We bought a soft-sided on at the local RV dealer (not a TM dealer). The top zips off to provide access, and there is a zip at the back for connections. Not expensive, but I can't remember the cost. I felt uncomfortable about having the bottles completely open to the rocks, etc and get blown up from the tow vehicle. The one we got works with 2-20 gal bottles, but they make them for 30 gal too.
Pam (half of the K&P Camping) :